In a laser beam printer, heretofore an F.theta. lens is used in combination with a rotary polygon mirror, to scan a photosensitive drum with the laser beam. One of the problems involved in deflecting the laser bean in a scanning mode, is that the scanning pitch becomes irregular because of the fall error of the reflecting mirror of the rotary polygon mirror. In order to solve this problem, a method has been proposed in the art in which an anamorphic optical system having a cylindrical lens surface and a toric lens surface is provided to give a surface fall compensating function to the F.theta. lens system (cf. Japanese Patent Application (OPI) No's. 126051/1979 and 144515/1982 (the term "OPI" as used herein means an "unexamined published application")), and in addition a method has been disclosed in which the curvature of a toric lens in the direction in which it shows a surface fall compensating function varies with the position of deflection (cf. Japanese Patent Application (OPI) No. 265615/1987). The section containing the optical axis of each of these lenses is arcuate both in the scanning direction and in the scanning vertical direction.
On the other hand, recently there has been a strong demand for the provision of a high precision laser beam printer. In order to fill this demand, it is necessary to reduce the diameter of the light spot formed on the scanning surface; that is, it is essential to form an optical system large in numerical aperture. The power of the F.theta. lens is, in general, greater in the scanning vertical direction than in the scanning direction, and therefore as the numerical aperture increases, the effect of the spherical aberration cannot be disregarded which is produced by the whole optical system, thus lowering the image-forming characteristic.
Furthermore, in the case where the printer is used with the numerical aperture changed, because of the spherical aberration, the position of the best image point is changed.